The trick here is to properly interpolate motion blur data based on the velocity vector of the current (center) frame unlike for static meshes motion blur where data is interpolated between frames, which will most likely fail for dynamic meshes. To achieve this, for now, the patch makes the very weak assumption that there's only gonna be two motion times (which
is the case in Cycles currently). A better approach is left for discussion.
Also the patch duplicates a bit of code from sync_mesh_motion. Would need to check on de-duplicating this.